The present invention relates to a receiving level control apparatus which controls a receiving level of a signal. More specifically, the present invention is directed to a receiving level control apparatus and a receiver, which are employed in a receiving circuit of a communication apparatus for performing a communication operation using a power line.
In power line communication apparatuses with employment of commercially available power lines as communication media, since leakage noises generated from power supply circuits of various sorts of consumer electric appliances may give adverse influences, high noise resistibility as to transmitting/receiving circuits of these power line communication apparatuses must be necessarily established. At the same time, impedance variation characteristics provided by electronic appliances which are connected to the power line communication apparatuses may constitute one of adverse factors. Under these reasons, multi-carrier communication systems have been proposed by which the same data is modulated on carriers over a plurality of frequency ranges, and data communication operations can be performed by utilizing such a frequency range with less adverse influence caused by noises, while avoiding such a frequency range that a large adverse noise influence is given, or another frequency range in which a large impedance variation is produced.
In the above-described multi-carrier communication systems, complex signal processing operations are required, so that longer data processing times than those of single carrier communication systems are necessarily required. As a consequence, in feedback control operations of AGC (Automatic Gain Control) circuits, processing times related to the feedback control operations must be necessarily shortened in order to improve communication performance, In the above-described feedback control operations, levels of input signals are controlled in such a manner that the signal levels become maximum within such a range that the controlled signal levels do not exceed maximum acceptable input levels of reception-purpose A/D converters.
Very recently, in order to improve communication efficiencies, shortening of symbol lengths of preambles in communication data is required. If the symbol lengths are shortened, then high-speed performance is furthermore required in AGC control operations which are performed in preamble portions. Accordingly, in order to realize high-speed communications, there is such a problem that response speeds in AGC feedback control operations become excessively slow.
In power line communication fields, electronic appliances are connected to and/or disconnected from power lines irrespective of communication operations to which power line communication apparatuses are connected. As a result, there is such a property that transmission characteristics of power lines corresponding to communication media are rapidly changed. Under such a property, there is another problem that in order to perform high-speed communication operations even when the transmission characteristics of the communication media are changed, high-speed AGC control operations are necessarily required.
FIG. 12 is a block diagram for showing a structural example of a receiving circuit containing an AGC circuit in a conventional communication apparatus. The receiving circuit contains a control section 51 which determines a gain of an AGC 52 based upon a reception analog signal “RD.” The control section 51 includes a first control circuit 51a and a second control circuit 51b, Also, the receiving circuit is arranged by employing the AGC 52, an analog/digital (A/D) converting circuit 53, a serial/parallel (S/P) converting circuit 54, a fast Fourier transforming (FFT) circuit 55, a tone selector 56, a primary demodulator 57, and also, a deframing circuit 58 (refer to, for example, patent publication 1) The AGC 50 amplifies a reception analog signal “RS” in an amplification factor under control of the control section 51. The fast Fourier transforming circuit 55 transforms the received reception analog signal “RS” from temporal axis data to frequency axis data. The tone selector 56 extracts a specific tone signal from the frequency axis data derived from the fast Fourier transforming circuit 55 based upon an instruction issued from the control circuit 51. The deframing circuit 58 outputs reception data “RD”
In an AGC process operation, the first control circuit 51a performs a saturation judging operation from an output result of the A/D converting circuit 53, performs a loop back control until a result of the saturation judging operation becomes unsaturated, or until an AGC table is used up to the last table (namely, reaches table having minimum gain), and first of all, provisionally determines a gain of the AGC. After the gain of the AGC is provisionally determined, the FFT circuit 55 performs a frequency analyzing operation, and the tone selector 56 extracts frequency data which is used in data transmission. Then, the tone selector 56 calculates a vector length with respect to a plurality of extracted frequency components. The vector length obtained in this calculation constitutes a signal component from which frequency noises other than the signal frequency is eliminated. The second control circuit 51b compares this signal component with a previously set threshold level, and if the calculated vector length is larger than the threshold level, then the second control circuit 51b defines the provisionally determined gain value of the AGC as an AGC gain value. To the contrary, if the calculated vector length is smaller than the threshold level, then the second control circuit 51b selects such a 1-stage higher value (amplification factor) than the provisionally determined gain value of the AGC of the amplification factors which can be selected from the AGC use table. As a result, the control section 51 performs the level control operation in such a manner that the level of the reception analog signal becomes a maximum level within such a range which does not exceed a maximum acceptable input level of the A/D converter 53.
Also, other conventional AGC control techniques have been proposed. That is, in the AGC control operations, while a present gain value approaches a target value of a gain, both a threshold value for comparing differences between the target value and present values with each other, and an evaluation time period for performing the comparisons are changed. In this conventional technique, when the present value is converged to the target value, the evaluation period is prolonged, so that the gain control operation can be carried out in a higher efficiency.    [Patent Publication 1] JP-A-2002-353813    [Patent Publication 2] JP-A-2003-32057
In the arrangement of the conventional technique as disclosed in the above-described patent publication 1, when the AGC gain control operation is carried out, the gain can be merely changed every only 1 step during a predetermined constant time period. As a result, even when the AGC gain control operation is carried out in the highest speed, a converging time of such a time period for a plurality of step numbers up to the target value is required, so that there is a limitation in high-speed gain control operations.
Also, in another conventional technique disclosed in the above-described patent publication 2 in which both the threshold value and the evaluation period when the AGC control operation is carried out are changed, there is such a problem. That is, when the present gain value reaches the target gain value, since the evaluation time period is long, if the value which should be controlled is changed when the present gain value is approximated to the target value, then the AGC control operation can be hardly carried out in high speeds.
The present invention has been made to solve the above-described problems, and therefore, has an object to provide a receiving level control apparatus and a receiver, which can be realized with employment of a simple arrangement while a complex processing operation is not employed, and which can perform a high-speed AGC operation with a high sensitivity.